This invention relates to a power amplifier in which a class B amplifier is employed as a current source in order to improve the efficiency of the class A amplifier, and which is so designed that an unnecessarily high voltage is not applied to the class A amplifier when a problem occurs in the class B amplifier.
In order to improve the efficiency of a class A amplifier, a circuit as shown in FIG. 1 has been proposed. In FIG. 1, reference characters A.sub.1 and A.sub.2 designate drive stages. The drive stage A.sub.1 operates to drive transistors Q.sub.1 and Q.sub.2 in a class A amplifier. The output of the drive stage A.sub.1 is applied to the bases of the transistors Q.sub.1 and Q.sub.2, the emitters of which are connected to an output terminal OUT, and the output terminal OUT is grounded through a load R.sub.L.
The collector of the transistor Q.sub.1 is connected to a DC source +E.sub.2, and the collector of the transistor Q.sub.1 is connected to a DC source -E.sub.2. The connecting point of the DC sources +E.sub.2 and -E.sub.2 is connected through a feedback circuit .beta. to an input terminal of the drive stage A.sub.2.
An input IN is applied to the input terminals of the drive stages A.sub.1 and A.sub.2. The output of the drive stage A.sub.2 is applied to the bases of transistors Q.sub.3 and Q.sub.4 which form a class B amplifier. The emitter of the transistor Q.sub.3 is connected to a DC source +E.sub.1, and the emitter of the transistor Q.sub.4 is connected to a DC source -E.sub.1.
The DC sources +E.sub.1 and -E.sub.1 are connected in series with each other, and the connecting point of the DC sources +E.sub.1 and -E.sub.1 is grounded. The collector of the transistor Q.sub.3 is connected to the DC source +E.sub.2 ; i.e., it is connected to the collector of the transistor Q.sub.1. The collector of the transistor Q.sub.4 is connected to the DC source -E.sub.2 ; i.e., it is connected to the collector of the transistor Q.sub.2. The voltages of the DC sources +E.sub.2 and -E.sub.2 are lower than those of the DC sources +E.sub.1 and -E.sub.1, respectively. In practice, alternating current is rectified and is then smoothed with capacitors C.sub.1 and C.sub.2 as shown in FIG. 2, to obtain the DC sources +E.sub.2 and -E.sub.2.
In FIG. 2, reference character T.sub.2 designates the secondary winding of a power transformer. Both ends of the secondary winding T.sub.2 are connected through diodes D.sub.1 and D.sub.2 to the capacitors C.sub.1 and C.sub.2, respectively. The capacitors C.sub.1 and C.sub.2 are connected in series with each other, and the connecting point of the capacitors C.sub.1 and C.sub.2 is connected to the center tap of the winding T.sub.2.
When a problem occurs in the circuit in FIG. 1: such as when the emitter and collector of one of the transistors Q.sub.3 or Q.sub.4 of the class B amplifier are short-circuited, the connecting point a of the DC sources +E.sub.2 and -E.sub.2 is brought into contact with the other elements or the class B amplifier oscillates, then for instance a current I.sub.1 flows to charge the capacitor C.sub.1.
Since the rated voltage of the capacitor C.sub.1 is low, the capacitor C.sub.1 is over-charged in such a case. The same thing can be said to occur on the negative side.
Thus, when, in the case where the class A amplifier employs the class B amplifier as a current source, a problem occurs in the class B amplifier, then an over-voltage is applied to the capacitors C.sub.1 and C.sub.2 forming the DC source of the class A amplifier, thus adversely affecting the class A amplifier.